1. Field of the Invention
The present invention relates to a supporting device for supporting an optical element, an optical apparatus, an exposure apparatus, and a device manufacturing method.
2. Description of the Related Art
An exposure apparatus exemplified by a semiconductor exposure apparatus is an apparatus that transfers a pattern formed on an original plate (e.g., reticle) onto a substrate (e.g., silicon wafer). During pattern transfer, a projection optical system is employed for imaging the pattern on the reticle onto a wafer. In order to produce a highly integrated circuit, a high resolving power is required for the projection optical system. For example, in a projection optical system for the semiconductor exposure apparatus, the uniformity of various properties related to glass material and film is required in order to significantly suppress aberration. Consequently, the processing accuracy and the fabricating accuracy of the surface shape of a glass need to be enhanced. A lens barrel for supporting a glass used for a lens is generally formed of a metal or the like which is a different material from a glass.
FIG. 11 is a perspective view illustrating the state in which a conventional supporting device is cut along its diameter, which shows the concept of a lens barrel structure. In FIG. 11, a lens 101 and a lens 102 are fixed to a metal frame 103 and a metal frame 104, respectively. The metal frames 103 and 104 are stacked in a cylindrical supporting member 105 by sandwiching spacers 106 and 107 for air gap adjustment, respectively, and are fixed by screw rings 108 and 109, respectively, by pressing from above.
However, in the lens barrel structure described above, the shape change of the lens or the lens barrel component due to changes in temperature of the environment may change the aberration. In particular, in the exposure apparatus that employs a short wavelength light source, a glass such as quartz or fluorite is used for a lens. However, the material of a lens, a lens barrel component for supporting the same, and the like have different coefficients of thermal expansion. Hence, these components cannot be uniformly expanded or contracted freely without being affected by other members. Consequently, the shape of the surface of the lens may change due to changes in temperature of the environment and the like, and thus the deformation has an effect on the aberration of a lens that cannot be ignored.
Solutions for solving the problem include the techniques disclosed in Japanese Patent Laid-Open Nos. 08-68899 and No. 10-96843. In the technique disclosed in Japanese Patent Laid-Open No. 08-68899, an optical element is supported by an elastic member that elastically deforms in a direction perpendicular to the optical axis of the optical element. Also, in the technique disclosed in Japanese Patent Laid-Open No. 10-96843, an optical element is supported by a non-elastic member that is pivotable about an axis parallel with the optical axis. In either case, the strain and stress of the optical element can be reduced even when the optical element and the metal frame expand or contract due to changes in temperature of the environment, whereby the optical element can be stably supported.
However, in the prior art such as the techniques disclosed in the patent documents described above, for example, in the technique disclosed in Japanese Patent Laid-Open No. 08-68899, it is a concern that the birefringence is likely to occur. The cause of the birefringence is that a restraining force is insufficient since an adhesive for fixing the optical element is circumferentially dispersed around a circumference of the optical element or because the distribution of pressure applied to the optical element is not uniform when the optical element is directly connected to an elastic member. Also, in the technique disclosed in Japanese Patent Laid-Open No. 10-96843, a pivoting member is directly joined to an optical element, whereby the optical element may receive a local couple on its joint section. This is not preferable because this may cause changes in the shape of the optical element.
In view of the above, the optical element is generally fixed to a ring-shaped supporting member having a linear expansion coefficient close to that of the material of the optical element by adhesion around its entire circumference, and the ring-shaped supporting member is then supported by an elastic member or a pivoting member. However, as disclosed in the patent documents described above, the displacement direction of the elastic member or the pivoting member for supporting the optical element is perpendicular to the direction of the optical axis, resulting in creating new difficulties. In the actual industrial products, it is difficult to dispose the center of gravity of the optical element, the sites for supporting the optical element in the gravitational force direction, and the position of the elastic member or the pivoting member on the same plane. Consequently, when the optical element and its supporting member expand or contract due to changes in temperature of the environment, this expansion or contraction is not uniform.
FIG. 12A is a schematic side view illustrating a case in which the center of gravity G of the lens, the supporting point A of a portion (ring-shaped supporting member) that supports the lens in the gravitational force direction, and the elastic member K shown in FIG. 11 are not positioned in a co-planar arrangement. FIG. 12B is an exaggerated side view illustrating how the lens and the ring-shaped supporting member expand due to a rise in temperature of the environment. When the lens and the ring-shaped supporting member independently expand due to a rise in temperature of the environment, the elastic member K is deformed in the direction of elasticity (the x direction in FIGS. 12A and 12B) so as to alleviate distortion caused by the expansion. However, since the elastic coefficient is not zero, a resisting force for suppressing the expansion is produced. Also, even when the elastic member K shown in FIG. 12 is a pivoting member disclosed in Japanese Patent Laid-Open No. 10-96843, the resisting force for suppressing the expansion is produced in a similar manner due to the friction of the pivoting section. By this means, the difference in the amount of deformation occurs between the section at which the elastic member K is provided and the vicinity of the lens supporting point A that is remote from the section in the z direction. As a result, the lens supporting point A is displaced in the +z direction (upward direction in FIG. 12B), and thus the lens is displaced in the +z direction (the displacement amount is shown by δz in FIG. 12B), resulting in a decrease in optical performance. In other words, the optical element is displaced in the optical axis direction depending on the rigidity of an elastic member used for relaxing the deformation of the optical element, upon a temperature change, resulting in the decrease in optical performance.